Equipment shelves of telecommunication service providers often contain communication circuit performance monitoring equipment, such as (DS3-associated) network interface units (NIUs). These devices are used to identify problems of a (digital) communication circuit to which they are coupled, owing to the ability of an NIU to be looped up and down, monitor line conditions, and execute performance monitoring duties. However, due to the possibility of an NIU card failure or some external event such as a power interruption, adding an NIU to the monitored circuit constitutes a potential failure point. As a consequence, it has been proposed to incorporate by-pass relay circuitry into the NIU card, as shown diagrammatically in FIG. 1.
In accordance with this NIU by-pass implementation, signal paths through respective network and customer transceiver circuits 11 and 13 of an NIU circuit card 10 are coupled to a first set of connection terminals 21, 26 and 31, 36 of a pair of associated relay-controlled switches 20 and 30. These switches have their contact arms 22, 24 and 32, 34 coupled to network line node pair 23, 27 and customer line node pair 33, 37. The switches are arranged to be controllably switched between the connection terminals 21, 26 and 31, 36 and connection terminals 25, 28 and 35, 38 by selective operation of a common relay coil 40. Energization current for the relay coil 40 is derived from a prescribed energizing DC voltage (e.g., 5 VDC) supplied by a DC—DC converter 45, that is coupled (through a fuse 47) to receive a prescribed DC voltage (e.g., −48 VDC) from a separate power supply path.
When energized (by the DC power supply 45), the relay coil 40 causes its contact arms to couple network equipment circuit paths 41, 51 to the NIU's network transceiver circuit 11 and customer equipment circuit paths 43, 53 to the NIU's customer transceiver circuit 13. In the event of a loss or absence of power to energize the relay, (the condition depicted in FIG. 1), the relay's contact arms by-pass the NIU transceiver components and, instead, connect the network circuit paths 41, 51 to the customer equipment circuit paths 53, 43, respectively, so as to minimize an interruption of (DS3) service.
Now, although the NIU-associated switching architecture of FIG. 1 will perform its intended by-pass functionality should there be a power supply failure, because the NIU by-pass relay 40 is physically installed on the same card containing the NIU components, the monitored (DS3) circuit will be undesirably interrupted should it become necessary to remove a damaged or malfunctioning NIU card from its housing or shelf. Moreover, as long as the power supply voltage is available to operate the relay, the architecture shown in FIG. 1 will not provide NIU by-pass, even if the NIU fails to work properly. In other words, in the prior art circuit architecture of FIG. 1, the only way to by-pass the NIU is to interrupt or remove the power supply.